Problem: Solve for $r$ : $30 + r = 18$
Subtract $30$ from both sides: $ \begin{eqnarray} \\ 30 + r &=& 18 \\ \\ {-30} && {-30} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{30 + r} &\hphantom{=}& \hphantom{18} \\ r &=& 18 {- 30} \\ \end{eqnarray} $ Simplify. $ r = -12$